Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme, which has found favour for use, for instance, in optical-based, wire-based (e.g. ADSL) and wireless-based data transfer applications (e.g. IEEE 802.11, WiMAX, UWB, LTE).
This is mainly due to its capability to provide high data rate transmission and also its robustness to multipath delay spread.
However, an OFDM-based communication system is much more sensitive to synchronisation errors than a single carrier system such as GSM communication system.
In fact, the channel through which an OFDM signal is transmitted, such as a time-varying channel, may introduce some noise and unknown signal time delay which may lead to synchronisation mismatches between a transmitter and a receiver of the OFDM-based communication system.
Therefore, time synchronisation is one of the essential functionalities of an OFDM-based communication system which needs to be one of the first operations performed by the receiver when receiving an OFDM signal.
Time synchronisation at the receiver aims at properly delineating a stream of OFDM packets representative of the OFDM signal, in order to avoid inter-symbol interference (ISI) by determining where each OFDM packet starts.
Usually, determining the time of start of an OFDM packet within a stream of OFDM packets representative of an OFDM signal is achieved by performing some operations on one or more of the first OFDM packets.
These operations may comprise the three following steps:                determining a coarse estimation of the time of start of the subsequent OFDM packets by correlating the incoming one or more of the first OFDM packets based on a plurality consecutive copies of a short training sequence;        determining a refined estimation of the time of start of the subsequent OFDM packets by correlating the incoming one or more of the first OFDM packets based on a plurality of consecutive copies of a long training sequence comprising more time-domain samples than the short training sequence; and,        determining the time of start of the subsequent OFDM packets based on the refined estimation results.        
However, in conventional time synchronisation techniques, determining where each OFDM packet starts may takes time since it may only be established based on a combination of the received short and long training sequences.
In fact, the correlation results obtained based upon the short training sequence taken alone are not completely reliable but only “coarse”.
This is mainly due to the fact the short training sequence has a short duration and thus contains few time-domain samples which may be very sensitive to the noise introduced by the channel through which the OFDM signal is transmitted.
This explains why a “fine” estimation is performed in a second step after determining the “coarse” estimation.
However, such conventional techniques do not comply with OFDM-based communication systems standards wherein time synchronisation is required to be determined as early as possible (e.g. IEEE Standard for WLAN “ISO/IEC 8802-11:1999/Amd 1:2000(E)”, p 7 (Section 17.3.2.1), p 12 (Section 17.3.3, FIG. 110) or p 30).
To summarise, determination of the time of start of each OFDM packets forming an OFDM signal as early as possible is not adequately addressed by known techniques.